a. Field of the Invention
This invention relates to an improved method and apparatus for the inactivation of microorganisms and reduction of biofilm in a self-contained potable water supply, or on the interior surfaces of devices used in the system. This is accomplished in part by installing one or more (UV) germicidal lamps inside at least one and preferably two or more of the following components: potable water tank, faucet, filter, water heater, humidifier. Additional microorganism inactivation and biofilm reduction can be provided through the use of a titanium dioxide, tin dioxide or other UV-activated antibacterial liner on the walls and welds of the components. The potable water supply may be installed in a remote portable structure or in a mobile vehicle such as a train, boat, bus, motor home, aircraft, and the like.
b. Background
Contamination of vehicular and other self-contained water systems is a significant ongoing problem. The World Health Organization estimates that up to one half of vehicle travelers suffer from some type of disease within a week of their trip.
A self-contained potable water supply typically consists of a water tank, a water treatment device, a method of moving the water, piping, a faucet and other water supply devices such as a humidifier. While the water may have started out as high quality drinking water, it soon deteriorates when placed in contaminated tanks and flowed through contaminated piping and devices. Bacteria and biofilm can grow in any of these devices. The moist metal or plastic surfaces are prime breeding areas for bacteria and viruses. The biofilm builds up on the surface until a portion of it sloughs off, eluding normal disinfection methods.
Small self-contained water supplies are often found in remote areas, as in cabins or other small buildings, or on vehicles such as yachts, boats, trains, busses, motor homes and aircraft. These systems store potable water for residents, crew or passengers and can easily become contaminated. Many people do not have a full understanding of bacterial growth. To avoid saving “stale” water in the storage tank, the water is usually drained at or near the end of a trip. The internals of most of the water supply components are consequently subjected to moist air for many hours until the surface dries. Unfortunately, bacteria, viruses and biofilm thrive in these conditions.
Furthermore, the water is often obtained from wells or other reservoirs where quality cannot always be assured. Reference is made to the City of Milwaukee where the city water supply was contaminated by Cryptosporidium in 1993. The result was more than 400,000 people sick and over 100 dead. In Las Vegas, the next year, 19 people died of the same pathogen. A serious problem exists that could become disastrous if a city's water supply should become infected and consequently the potable water supplies in such vehicles or other self-contained potable water supplies are filled with contaminated water. The increased potential for intentional contamination by terrorists, e.g., using anthrax or other pathogens, has heightened this concern for both civilian and military users.
Biofilm is a major mode of self-protection for many types of bacteria. The bacteria grow and attach to the surface of piping and fixtures by producing a sticky matrix. Algae and protozoa may also contribute to the colony. As well as water, the biofilm matrix is made up of lipopolysaccharides and acidic exopolysaccharides excreted by the bacteria. The exopolysaccharide and water gel when sufficient calcium ions replace the acidic protons of the polymers. Chlorination is not very effective in reducing bacteria hidden behind the protective biofilm. Bacteria counts taken from the water stream only measure the floating microorganisms, not the number that exists in biofilm attached to the walls of supply components and piping.
It has been discovered that bacteria start the colonization process at a specific location by communicating with each other (“Community Structure and Co-operation in Biofilms”, Cambridge University Press, 2001; Scientific American Volume 285, July 2001). Thus, to prevent the start of a biofilm colony, the surface of components and piping must be treated so that either the ability to communicate is blocked or attachment by bacteria is prevented.
Mutually beneficial species like sulfate-reducing bacteria (e.g. Desulfovibrio vulgaris) and sulfate-based bacteria (e.g. Thiobacillus) cooperate to cause accelerated rates of iron corrosion under a biofilm. Desulfovibrio vulgaris produces hydrogen sulphide as a waste product. Thiobacillus uses hydrogen sulphide for nourishment, producing sulfuric acid. Both are corrosive and can damage the surface of some iron-based materials. Under the right conditions, biofilm can grow quickly and then slough off pieces into the water supply.
Cryptosporidium is a cyst that is spread onto surface waters that eventually drain into water reservoirs. Chlorination is not very effective in the elimination of Cryptosporidium, however UV disinfection has proven to be effective even at moderate doses.
As well as pathogenic bacteria viruses may also be present, the latter being the smallest of waterborne pathogenic microorganisms. These viruses, such as those producing infectious hepatitis, poliomyelitis, meningitis, and gastroenteritis, can hide under biofilm among various bacteria species.
Air-borne bacteria and viruses can also contaminate water tanks and humidifiers connected to the potable water system. Legionella bacteria came to prominence with the 1976 Philadelphia convention of the American Legion. Several people died of the harsh infection associated with legionella pneumophila. Poor cleaning of the biofilm in the air conditioning system was attributed as the cause of the bacterial buildup. The legionella pneumophila often grows in conjunction with, and is protected by, biofilm. The air blowing through coils and humidifiers can carry bacteria and viruses through air ducts in a vehicle to where humans are located.
A wide variety of methods have been used over the years to disinfect water, including chlorination in particular. However, recent research has provided strong evidence of health disadvantages associated with using chlorine as a primary means of disinfecting drinking water; for example, the US Environmental Protection Agency has advised that certain chlorine byproducts created during water treatment are carcinogenic. Furthermore, chlorine is not effective in removing biofilm from a water supply and harms the environment.
While also effective as a disinfectant, the use of ozone is discouraged onboard vehicles because of the corrosive nature of the gas, its odor and its harmful nature to humans when the concentration is not properly regulated.
Ultra-violet light (UV) is a very effective disinfecting agent with few, if any, side effects. UV exists in nature, radiating from the sun. UV light adds no chemicals to the water, and therefore creates no health, taste or odor problems. It is well established that germicidal lamps emitting UV light in the 254 nm range are effective at disinfecting most bacteria (including anthrax), viruses and cysts. UV is a preferred method of primary disinfection for water supplies as compared to injecting a moderate level of chlorine (over 1.0 ppm). Furthermore, contrary to ozone, excessive application of UV light at the 254 nM germicidal wavelength does not harm either the potable water or humans drinking the water.
Current practice for UV treatment of self contained water systems is to connect an individual ultra-violet disinfection unit (UVDU) to the pipe exiting the water tank. Installed at this location the UVDU is called Point-of-Entry (POE) or inline disinfection. Although conventional, this method has many deficiencies, including the following:    a) The POE UVDU only disinfects the water flowing through it. It is not effective in disinfecting or removing biofilm in the water tank, filters, accumulator, faucets, humidifier, and so forth. It has been determined by researchers that circulation of UV treated water is not enough to prevent the growth of biofilms in low-flow portions of the piping, rough welded seams, nor in non-UV irradiated components.    b) The POE UVDU is an additional component that takes up extra space that is usually very valuable in limited-size vehicles.    c) The POE UVDU is extra weight that must be carried at the cost of extra fuel and/or reduced range or passenger/load capacity.    d) The POE UVDU and filter can create a pressure drop of more than 10 psi, which is substantial in many low head-pressure water supplies.    e) The POE UVDU takes a considerable amount of time to treat all of the potential bacteria in the water within the supply if the treated water is circulated back to the tank and mixed in with the contaminated water. This is because of the dilution factor of the tank water. To theoretically disinfect all of the bacteria in the tank water typically would take more than a day of circulation.    f) If the POE UVDU fails, system reliability is compromised. Greatly increased reliability would be provided with separate, redundant UV treatment lamps.    g) The POE UVDU can overheat if no water flows through it at least every twenty to thirty minutes, reducing the efficiency of UV output.
Moreover, when non-cylindrical enclosures are used for disinfection devices, a situation can arise where unequal UV intensity exists at different distances from the UV radiation source. If the UV intensity is designed to be greater than necessary, power is wasted and excess heat is created. Another facet of this problem is where rough surfaces, such as welded joints, on interior surfaces of the water tank or other devices harbor bacteria from the direct UV rays, so that the disinfection is not fully effective.
As an ancillary aspect, a common method employed in the prior art for keeping the tanks, piping and apparatus of self-contained water supplies from freezing is to install an electrical heater in the tank and/or have electrical heaters wrapped around the piping. A UV lamp, or multiple lamps, of sufficient size offers a potentially superior solution, as these are able to accomplish both heating and disinfection purposes at a lower power requirement.
c. Related Art
A variety of devices are shown in the prior art that seek to treat water or water vapor with UV light. However, as noted above, in the bulk of these devices the water is exposed to the UV radiation only as it flows through one portion or point in the system. As a result, the treatments are at most only partially effective and the devices share many of the deficiencies discussed above as well as other drawbacks.
Certain other of the prior art illustrates the marginally effective applications of UV treatment in air humidification devices:
Certain of the prior art is directed to UV disinfection prior to storage in or circulation through a tank.
Coviello, et al. (U.S. Pat. No. 4,179,616) describe use of a UV device for disinfecting water or other fluids, where the treated fluid is then sent to a holding tank. Valadez (U.S. Pat. No. 4,969,991) teaches a water disinfection system for vending machines and the like, in which water is purified before placing it into a holding tank, and then circulated past a UV disinfection unit and back to the tank. Wang et al. (U.S. Pat. No. 5,256,299) relates to a large-scale, industrial process for treating contaminated liquid to remove volatile organic compounds. UV treatment only occurs in the mixing section, and in any event the system is by its nature massive and wholly unsuited to portable or vehicular use. Kool et al. (U.S. Pat. No. 6,245,229) disclose a single point-of-use water treatment system unit for home or office use. This is a modular unit having a filter chamber attached to a UV chamber adapted to be placed in-line. None of these references are concerned with or effective at reducing biofilm on the interior of the tank, piping, or the other water supply components.
Thorpe (WO 00/75081 A1) describes a UV device primarily intended for disinfecting potable water in vehicles. This POE UVDU also has many of the limitations described above. While effective for disinfection of water that passes through the unit, it will not control the bacteria, viruses and biofilm that grow in the other devices and piping that are part of the potable water supply.
Walker (Cdn. Pat. No. 2,345,995) discloses a continuously circulating water system that has a reduced sensitivity to the temperature of the surrounding environment, specifically to prevent freezing of the water lines. Walker employs POE water treatment devices, comprising a filter and/or an ultra-violet disinfection device, as the only means of disinfection and therefore shares many of the above deficiencies, e.g, the apparatus will not control the bacteria, viruses and biofilm that grow in the other devices that are part of the potable water supply system.
Levine, et al (U.S. Pat. Nos. 5,677,982 and 5,859,952) teach a humidifier apparatus that only utilizes UV light for disinfection of water in the flow to the heater. This approach does not recognize the dangerous buildup of bacteria and biofilm in the airflow duct where water or mist is introduced. Air-borne bacteria and viruses such as Legionella pneumophila can often be present at this type of air/water interface. Whitely (U.S. Pat. No. 4,362,090) describes an air-circulating device placed on the floor of a room with a pair of ducts extending to the ceiling. A humidifier is disposed within the base of the device and uses a sponge to soak up water. Air is passed through the sponge and is humidified. A UV lamp is placed in one of the ducts in an effort to irradiate the airflow. However, no disinfection of the sponge or tank is accomplished, thereby allowing growth of bacteria and biofilm.
Accordingly, there is a continuing need for improved methods and apparatus for improving the quality of water in a self-contained potable water supply and reducing biofilm growth. Unexpectedly, a method and apparatus have been discovered that remove most of the impediments of current potable water supply systems and overcomes the limitations a POE UVDU.